Magnetic lenses are generally known and are often employed to focus a beam of charged particles e.g., electrons. One major use of a magnetic lens is in analytical instruments such as electron microscopes, which utilize focused electron beams to stimulate a reaction from a sample that is being observed or analyzed. In such instruments the magnetic lens is useful as an objective, or final focusing lens. That is, the objective magnetic lens is the last focusing element prior to the beam impinging on the sample. As such, the stability of the beam position as affected by the magnetic lens is quite critical to achieve an accurate characterization of small features on the sample.
FIG. 1 is a cross-sectional, schematic drawing of a conventional magnetic lens assembly 100. Lens assembly 100 includes upper section 110 (which comprises an electron detector assembly), coil 115, and pole assembly 120, which includes upper pole piece 122, outer pole piece 24, inner pole piece 126, cooling ring 127, and insulator 129. Substantially collaring the ring-shaped coil 115, the pole pieces form a magnetic circuit around the coil for carrying magnetic flux when current flows through the coil. An air gap 131 proximal to the sample to be analyzed is disposed between the inner and outer pole pieces 124, 126. The centerline of the lens corresponding to the beam path is indicated at 105. The primary electrons are focused by the magnetic field B at the end of the lens near the air gap. Because the air gap 131 has significantly higher reluctance than the combined pole pieces, “fringing” occurs at the air gap causing the magnetic field B to bow, as shown, toward the sample in a desired annular shape for focusing incoming electrons approaching the sample.
One feature of conventional analytical instruments employing an electron beam is that the energy of the electron beam is variable over a range of energies consistent with the range of materials to be characterized. For example, in an Auger analysis, the energy of the electron beam determines the depth of penetration and the range of Auger excitation of the atoms of the sample. Since the energy necessary to produce Auger electrons varies from atom to atom, a beam generator, to be effective, should be capable of producing electrons of various energies. In addition, the beam energy should be variable when the sample under analysis is an insulator because if the beam energy is high enough, the sample can become electrostatically charged. Such charging can be avoided by reducing the beam energy, sometimes to as low as 0.5 KeV. Another instance that mandates a variable beam energy is where an X-ray detector is used. In order to produce atomic emissions in the X-ray spectrum, the energy of the charged particle beam must be quite high, for example, on the order of about 30 KeV. Thus, to be practical, the energy of the electron beam should be variable over a broad energy range. As one might expect, however, when the beam energy or beam size is changed, the strength of the lens magnetic field typically must also be adjusted in order to ensure that the beam having the changed energy remains focused on the same point of the sample as the beam of previous energy.
The magnitude of the magnetic field generated by the lens 100 is proportional to the number of coil-turns multiplied by the current in the coil 115 (measured in amp-turns, At). Thus, by changing the current in the coil, one can vary the value of the magnetic field to match the energy of the beam. However, because the lens coil is typically made of wire, which has electrical resistance, electrical power (heat) is generated in the coil. (The amount of power P=R·I2). For example, in a typical coil, 1140 Amp-turns may be required. This can translate to about 15.5 W, which is a significant amount of heat, especially to be dissipated in the confined environment of a magnetic lens coil. The heat emitted from the lens coil will raise the temperature in the lens pole assembly, which causes it to expand. (Likewise, a decrease in field strength causes the temperature to fall and the pole assembly to contract.) This is problematic because the exact position of the lens pole pieces with respect to each other and the centerline of the electrons define the position where the electrons are focused on the specimen. Thus, changes in field strength affect the focusing characteristics of the lens coil. This can impose a delay in operating the device after changing the field strength so that the lens coil has sufficient time (e.g., one to seven hours) to thermally stabilize. Accordingly, a major consideration in designing a magnetic lens is the removal and dissipation of the thermal power generated by it in order to speed up this delay. Unfortunately, the reaction times of available thermal dissipation mechanisms are typically inadequate to readily keep pace with thermal changes caused by changing the strength of the magnetic field. While the delay may be decreased, time must still be wasted waiting for the lens to “settle.”
One solution has been to use a multi-coil design that generates constant heat power over the operating magnetic-field strength range. Such a design is shown in U.S. Pat. No. 4,345,152 to Gerlach, which is hereby incorporated by reference into this specification. The lens is composed of a pair of sub-coils substantially in parallel with a common terminal. They are configured so that the currents travel in opposite directions around a common core. This is achieved with a pair of adjacent, but electrically insulated conductors wrapped in tandem (bi-filair) around the core. The magnetic fields created by each coil are proportional to their respective currents but offset one another since they have oppositely directed currents. In turn, their generated magnetic fields effectively offset each other. The overall lens field strength is minimized when their magnitudes are the same, while the overall field strength increases as the difference between the individual current magnitudes increases, but the power generated by each coil is simply a function of the magnitude of its current squared. Thus, by varying the subcoil current difference but keeping constant the sum of their I2R magnitudes, the generated power can be held constant while the effective lens magnetic field strength is varied. Unfortunately, one drawback with this approach is that the overall response of the magnetic lens is fairly slow. That is, in order to attain the desired range of magnetic field strengths, both coils have a significant number of turns thereby giving them relatively large self-inductances. In fact, they can be large enough to inhibit field strength changes that are fast enough for such practices as dynamic focusing.
Accordingly, what is needed is an improved magnetic lens that can remain thermally stable and at the same time be sufficiently responsive over a range of operating magnetic field strengths.